Sustainable removal of Hg(II) by sulfur-modified pine-needle biochar

Sulfur-modified pine-needle biochar (BC-S) was produced for the removal of Hg(II) in aqueous media via post-pyrolysis S stream exposure. Fourier-transform infrared spectroscopy, elemental analysis, and X-ray photoelectron spectroscopy confirmed the addition of S(0) groups on the surface of BC-S. Hg(II) adsorption on BC-S was best described by the Freundlich isotherm with a K_F of 21.0 mg L g^-1 and a pseudo-second-order adsorption kinetics model with a rate of 0.35 g mg^-1 min^-1. Hg(II) removal on BC-S was found to be an endothermic process that relied on C-Hg and S-Hg interactions rather than reduction by S(0) groups. The adsorption increased with increasing solution pH and decreased with increasing dissolved organic matter concentration, but was unaffected by increasing salt concentrations. BC-S showed a maximum of 3 % S leaching in aqueous media after 28-d exposure time, and exposure to aqueous media did not convert Hg(II) to elemental Hg. Overall, BC-S exhibited superior Hg(II) removal performance over unmodified BC, thus having potential applications in natural water and wastewater treatment with no significant threat of secondary pollution.